1. Field of the Invention:
The invention generally relates to video decoders and more specifically to composite video decoders and FM demodulators for use in decoding the color information of a SECAM (Sequential Couleur Avec Mémoire) composite video signal.
2. Description of Related Art
There is a large surge in the use of digital video devices today. Examples include: digital televisions, LCD (Liquid Crystal Display) TVs (televisions) and monitors, DVD (digital versatile disk) recorders, personal video recorders, PC (personal computer) video cards, video capture and streaming applications, and video conferencing. In many cases, these units need to receive an analog video signal, which may be one of the composite signals, such as NTSC (National Television Standards Committee), PAL (Phase Alternating Line) or SECAM; S-video; component video or RGB (Red-Green-Blue). It is then desirable to produce the proper digital output, such as eight or ten bit ITU-R (International Telecommunications Union-Radio Communication) BT (Broadcast Television) 656. It is preferred that all the video decoding be done in a single chip for all of these formats. The decoder not only has to handle composite signals, which means it must be able to determine the chroma and luma values, but it also must handle vertical blanking interval (VBI) data and handle VCRs (video cassette recorders), which frequently have unstable timing signals.
Although a number of such systems have been developed, it is always desirable to improve the output and capabilities of the particular video decoder. For example, it is desirable to support as many video formats as possible while maintaining a certain level of circuit simplicity, both for cost and reliability purposes. Unlike NTSC and PAL composite video, SECAM video includes color information that is frequency modulated. Thus to be able to decode SECAM video, a video decoder must include an FM demodulator. Historically, phase locked loops (PLLs) have been used for FM demodulation. However, these components are susceptible to various forms of electrical noise, and further add to the cost and complexity of the video decoder circuits.
Therefore a need exists in the art for a means of performing time-based FM demodulation to eliminate the need for the PLL. It would be further advantageous to perform the demodulation in the digital domain, so as to reduce the susceptibility of the circuitry to various forms of noise and error.